Introduction
First metatarsophalangeal (MTP-1) joint arthrodesis is a commonly performed procedure in the treatment of disorders of the great toe ( Gaudin et al., 2018 ). A good outcome following this procedure depends on two factors: mechanical stability of the bone during the postoperative period and sufficient blood supply to the bone ( Claes, 2021 ). The excessive transverse and opening displacements between the bones may delay or even suppress the bone formation process ( Augat et al., 2003 ; Claes et al., 2018 ). Therefore, the relative displacements between the bones and the stiffness of fixation systems are used as measures of the mechanical stability of the bone arthrodesis ( Claes, 2021 ). The successful bone union without hardware failure and bone fracture is necessary to achieve pain relief and functional improvement. The maximum load allows to assess the risk of failure of fixation system or bone fracture. The current works focus on new minimally invasive techniques of arthrodesis stabilisation strong enough to allow weight bearing without loss of the initial stiffness of the bone fixation ( Fuld et al., 2019 ; Hodel, Viehöfer & Wirth, 2020 ). Since the incidence of surgical revision after MTP-1 joint arthrodesis is not insignificant ( Gaudin et al., 2018 ), a medial approach with a medially positioned locking plate was proposed. This technique offers e.g ., protection of the vascular supply to the metatarsal head, excellent exposure, superior cosmetic result, safe distance from the dorsomedial cutaneous nerve and reinforcement of medial hallux correction in hallux valgus deformity ( Dayton, Glynn & LoPiccolo, 2001 ). A pilot clinical trial showed that MTP-1 joint arthrodesis is safe procedure and give clinical results comparable to those obtained with currently widely used techniques ( Kuik & Łuczkiewicz, 2021 ).

Significant reduction of interfragmentary motion ( Märdian et al., 2019 ) and faster fracture healing ( Horn et al., 2011 ) was demonstrated when an interfragmentary lag screw was used in a locking plate construct. The addition of a lag screw to the dorsal plate improves significantly the stability and strength of the bone fusion in the MTP-1 joint ( Buranosky et al., 2001 ; Politi et al., 2003 ; Campbell et al., 2017 ). Relative transverse displacements between bones close to 1.5 mm were reported for the medial approach in a study ( Witkowski et al., 2021 ), which may adversely affect the healing process ( Augat et al., 2003 ). We propose the use of lag screw as a method to minimize such deformation in the medial approach. So far there is no published study that quantifies the biomechanical performance of MTP-1 joint arthrodesis with a medially positioned locking plate and lag screw. Therefore, the purpose of the study was to investigate the effect of the lag screw on the stiffness and strength of the MTP-1 joint arthrodesis with the medial plate.

Numerical and experimental results from a previous study ( Witkowski et al., 2021 ) have suggested that the medial plate may provide better stability of the MTP-1 joint arthrodesis than a dorsally positioned locking plate due to the greater vertical bending stiffness of the medial plate. However, only one sample was tested for each technique and the results were compared to the weak dorsal plate construct in Witkowski et al. (2021) . Therefore, the secondary objective was to perform experimental tests on a larger number of samples and compare the results between the medial plate constructs and the dorsal plate with lag screw, described as the most stable fixation method ( Politi et al., 2003 ; Harris, Moroney & Tourné, 2017 ).